Why does the function always refer to one specific parameter? - c++

Question

In the following code:
int sum(int a=40, int b=20)
{
int result;
result = a + b;
return (result);
}
int main ()
{
int a = 100;
int b = 200;
int result;
result = sum(a, b);
cout << "Total value is :" << result << endl;
result = sum(a);
cout << "Total value is :" << result << endl;
return 0;
}
This produces:
Total value is : 300
Total value is : 120
Why does the:
sum(a)
add the (int a) in the 2nd block to the (int b) in the 1st block?
Im confused why the (b) value in the 1st block is used in (sum(a)), but the (a) value in the 1st block is ignored.

Answer 1

int sum(int a=40, int b=20) {
...
}
declares the parameters a to be 40 and b to be to 20, if not specified. This is a compiler service, so sum(a) becomes sum(a, 20) (b not specified). Similar to this, sum() becomes sum(40, 20). a and b in your method sum are default parameters.

Answer 2

In function sum you are using default arguments. That's why when you call
result = sum(a); // where a = 100 and the second parameter is ommited
in the function sum, the first parameter is take the value of this caller's a (= 100), and as the second parameter is absent from the caller's end, the default b (= 20) will be used as b. Hence the result is
100 + 20
= 120
As David Rodríguez suggested in the first comment, use different variables name (say sum (int x, int y)) for no ambiguity and better understanding.

Answer 3

In order to make it shorter:
int sum(int a=40, int b=20)
{
return a + b;
}
int main ()
{
int a = 100;
int b = 200;
cout << "Total value is :" << sum(a, b) << endl;
cout << "Total value is :" << sum(a) << endl;
return 0;
}
In the sum(a,b) both of the parameters have values => it does a+b => 100 + 200 which is 300.
In the sum(a) the second parameter is not set, the function use the default value (ie: 20) => a + 20 => 100 + 20 which is 120
The a & b you define in your main are not the one of the sum function

Answer 4

Well, you should read a bit about default parameters in C++. Where you are on it, I recommend you to research a bit about overloading, since they are somewhat related.
On the first call you do to the sum() function, you provide both parameter to the call, so the variables a and b ,that you declared, are used, hence you get 100+200=300. On the second call tho, you only provide one parameter, so the second one uses the default parameter value, i.e. a=100, b gets the default value (20), so you get 100+20=120.

Answer 5

When you write a function like
returntype Function_name(data_type some_value), then this is called default parameters.
For eg: if you write a function like,
int calculate_area(int lenght=20, int width=25)
Then when you call this function from main, you can either pass values to length and width like this,
int main() {
int a=50;
int b=60;
calculate_area(a,b);
}
Or you can call it like this...
int main() {
calculate_area();
}
See the difference, we are not passing any parameter, still it is a valid call to the function, because in this case.... the default values mentioned by you for length and width will be considered, which in this case is 20 & 25.
And about variables 'a' & 'b' in your code, looks like you are getting confused between name of the variables. Main() and Sum() are two different functions.
'a' of sum has nothing to do with 'a' of main. You will understand this when you will read how the variables are stored in stack and all.